1. Field of the Invention
The present invention relates to an electronic timepiece which utilizes the photovoltaic voltage such as a solar cell as a power supply and, more particularly, to a photovoltaic electronic timepiece which performs an alarm display when timepiece information becomes incorrect due to reduction in a power supply voltage.
2. Description of the Prior Art
Conventionally, cells such as a mercury cell or a lithium cell are used as a power supply of an electronic timepiece, e.g., an electronic wrist watch. However, along with development of a capacitor with a large capacitance, a solar cell has been used as a power supply. In an electronic timepiece which utilizes a solar cell as a power supply, the photovoltaic voltage of the solar cell is stored in a capacitor, and a terminal voltage of the capacitor is used as a power supply. However, similar to an electronic timepiece which utilizes a cell as a power supply, there is a known electronic timepiece of this type which utilizes the photovoltaic voltage as a power supply, in which a normal time display is switched to a display different therefrom, i.e., a modulated display when a voltage of a storage battery (a power supply voltage) is reduced below a voltage level for causing the timepiece to perform the normal display so as to inform a user of need for charge (as disclosed in, e.g., U.S. Pat. No. 4,219,999).
However, in the electronic timepiece of this type, when a user does not notice the modulated display and keeps using the timepiece without charging it, the voltage of the capacitor is further reduced, and then generation of a time base signal by a quarter crystal oscillator is stopped. In this case, the user notices abnormality of the timepiece because a time display device is also stopped, resulting in no problem. However, when the timepiece is charged to return to a modulated display state or to a normal display state by charging after the time base signal is stopped and the user does not know this fact, the user uses the timepiece without knowing that the timepiece has lost time corresponding to time in which generation of the time base signal was stopped since the time display device is driven as usual to display time. In addition, in the case of an analog electronic timepiece, the above problem due to reduction of the power supply voltage occurs not only when the oscillator circuit is stopped but also when a pulse motor for driving hands is stopped and then returned to normal state or when generation of the time base signal is stopped due to a factor other than voltage reduction of the capacitor and then returned to normal state.